Method for producing a metal-supported catalyst and catalyst substrate

ABSTRACT

The present invention relates to methods for producing metal-supported thin layer skeletal catalyst structures, to methods for producing catalyst support structures without separately applying an intermediate washcoat layer, and to novel catalyst compositions produced by these methods. Catalyst precursors may be interdiffused with the underlying metal support then activated to create catalytically active skeletal alloy surfaces. The resulting metal-anchored skeletal layers provide increased conversion per geometric area compared to conversions from other types of supported alloy catalysts of similar bulk compositions, and provide resistance to activity loss when used under severe on-stream conditions. Particular compositions of the metal-supported skeletal catalyst alloy structures can be used for conventional steam methane reforming to produce syngas from natural gas and steam, for hydrodeoxygenation of pyrolysis bio-oils, and for other metal-catalyzed reactions inter alia.

FIELD OF THE INVENTION

The present invention relates to methods for producing metal-supportedthin layer skeletal catalyst structures, to methods for producingcatalyst support structures without separately applying an intermediatewashcoat layer, and to novel catalyst compositions produced by thesemethods. Catalyst precursors may be interdiffused with the underlyingmetal support then activated to create catalytically active skeletalalloy surfaces. The resulting metal-anchored skeletal layers provideincreased conversion per geometric area compared to conversions fromother types of supported alloy catalysts of similar bulk compositions,and provide resistance to activity loss when used under severe on-streamconditions. Particular compositions of the metal-supported skeletalcatalyst alloy structures can be used for conventional steam methanereforming to produce syngas from natural gas and steam, forhydrodeoxygenation of pyrolysis bio-oils, and for other metal-catalyzedreactions inter alia. The interdiffused alloy surfaces optionally may beformed into bulk monolithic structures (before or after activation) andfurther treated in an oxidation step to generate adherent oxide layersthat provide intrinsic support surfaces for secondarily appliedcatalytic agents such as dispersed precious or platinum group metals.Metal substrate form factors such as fibers can be treated by themethods described herein to generate adherent thin-layer skeletalcatalysts and/or catalyst supports, which, in turn, can be fabricatedinto structures of high geometric-surface-area-to-volume ratios.

BACKGROUND

Intensified processing methodologies are used increasingly in place oftraditional chemical processing routes when small production volumes arewarranted or when portability of process equipment is desired. Forexample, large scale hydroprocessing of biofuels derived from localsources is impractical for isolated military units in remote locations.Instead, portable conversion units and/or units with small footprintsare needed. However large scale processing reactors are not easilydownscaled for such uses. Thus, the need exists for competent catalystsand corresponding reactor configurations that are suitable for on-siteprocess chemistry in small, modular units.

Simply operating established heterogeneous catalysts in compact reactorconfigurations at higher space velocities and at more severetemperatures than used in traditional processing will not necessarilyincrease productivity as needed. Under such modified conditions, massand heat transfer limitations attenuate maximum catalyst activity,especially when such processes are conducted in traditional reactordesigns. Consequently, intrinsically fast catalytic cycles alone affordno additional productivity benefits without addressing the mass and heattransfer limitations.

A variety of different shapes and styles of heterogeneous catalysts arecurrently available that seek to augment the ratio of geometric surfacearea to occupied reactor volume as a means of mitigating mass and heattransfer limitations. Preformed metallic scaffolding structures arepreferred over ceramic scaffolding structures when very thin walls orlow pressure drop is needed in densely packed channel structures such asthose desirable for intensified processing. In particular, microchannelconstructs, such as thin-walled metallic honeycombs, covered with aminimally thick catalytic layer, have been sought to decrease resistancebetween process fluids and channel walls thereby promoting rapidconvective heat and mass transfer as well as conductive heat transfer.

The ratio of surface atoms exposed to process fluids to total atoms of acatalytically active metal cluster is termed “dispersion” for thepurposes of this discussion. Catalyst support surfaces that enable highdispersion of applied catalytic agents are sought in the art. To reducerequired content of costly catalytic components and to achievesufficiently high dispersion to promote high catalytic activity perstructural unit, base metal or precious or platinum group metalheterogeneous catalysts usually are applied to catalyst supportmaterials, often composed of high surface area oxide powders. Typically,structured catalyst supports are produced by applying an intermediatewashcoat layer of inert metal oxide or aqueous hydroxide slurriesdirectly onto metal foils that have been preformed into a desiredscaffolding structure. A second application of precursors transformableto reduced metal clusters, e.g. alcoholic or aqueous solutions of metalsalts, is usually applied afterward. Alternatively, the metal salts canbe admixed with the oxide or hydroxide aqueous slurries and applied inone step to the metal substrate. Catalytically active metals or activecompounds are thereby distributed within the washcoat layer, rather thanin exclusive direct contact with bare metal substrate. The activematerial therefore sits on the surface of the intermediate washcoatlayer and is insulated from the underlying metallic substrate. As such,the washcoating is susceptible to damage, such as delamination, fromaggressive physical manipulation and/or intensified process conditionsbecause the coating is only weakly adhered to the underlying metalscaffolding structure. These catalyst structures usually are coatedafter physical forming of the scaffold to minimize damage to the curedcatalyst coatings that could result if mechanical processing were doneafter application of the washcoat.

The application of slurries to preformed substrates also can result inthe coating having a nonuniform distribution. Preformed substrates arenormally dip-coated or spray-coated with the washcoat slurry, and excessslurry is removed using an air knife. Excess slurry is difficult toremove from small crevices and corners, particularly in catalyststructures containing microchannels, and can result in varyingthicknesses of the washcoat throughout the catalyst structure, whichleads to a catalyst layer of varying thickness on the substrate.

When such a supported catalyst is used to accelerate an exothermicreaction, e.g. catalytic oxidation of entrapped soot particles or ofgaseous hydrocarbons, the varying thickness of the catalyst could resultin hot spots forming in the catalyst layer, which in turn can causemelting of the substrate or sintering of the active phase, therebyprematurely reducing dispersion and corresponding activity. Analternative to the use of high surface area supported base metals ascatalysts is the possibility for use of bulk skeletal metal aggregates,such as Raney metals, to prepare highly active catalysts. These skeletalmetal particles typically are used in slurry phase processing or, lesscommonly, in packed beds. The latter usually suffer from pressure dropor particulation problems in practical use. Small channel monolithstructures containing bound bulk skeletal metal catalysts, which inprinciple could generate a diminished pressure drop compared to packedbeds under high space velocity conditions, would be difficult and costlyto fabricate. Furthermore, under severe process conditions, such asencountered in steam methane reforming for example, bulk skeletal metalaggregates would rapidly deactivate due to surface sintering or easilydelaminate from their underlying scaffolding, if used.

SUMMARY OF THE INVENTION

Accordingly, the present invention addresses problems and expands theapplications in the prior art and, in a first manifestation, provides amethod of producing an intrinsically bound thin-layer skeletalcatalyst-coated metal foil or fiber with relatively uniform coatingthickness capable of physical manipulation into a highly activecatalytic monolithic structure, without requiring separate applicationof an intermediate washcoat or additional catalytic agents.

In particular, a first aspect of the present invention is a method formaking a catalyst, comprising the following steps:

(a) preparing a slurry comprising one or more metal (includingprealloyed) powders including aluminum;

(b) coating a flat metal substrate or a flattened mat of metal fiber ora flattened woven metal fiber assembly with said slurry;

(c) subjecting the coated metal substrate or coated metal fiber mat orcoated woven metal fiber assembly to heat under an inert or reducingatmosphere whereby at least one of the one or more metal powders meltsand interdiffuses into the surface of the flat metal substrate or metalfiber mat or woven metal fiber assembly;

(d) leaching the coated metal substrate or coated metal fiber mat orcoated woven metal fiber assembly obtained in step (c) in a causticsolution;

(e) bathing the coated metal substrate, coated metal fiber mat or coatedwoven metal fiber assembly obtained in step (d) in a chelating acidsolution;

(f) passivating the coated metal substrate, coated metal fiber mat orcoated woven metal fiber assembly obtained in step (e); and

(g) optionally abrading the surface of the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly obtained instep (f).

The coated metal substrate or coated metal fiber mat or coated wovenmetal fiber assembly obtained at the end of step (c) is physicallymanipulated into a desired form for the final catalyst either: afterstep (c) and before the leaching step; or after the passivating step.

In step (c) of the above-described process, the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly is heated toa temperature in the range of about 600-1100° C., preferably from about650-910° C., for a period of time of from about 0.2 to 4 minutes,preferably from about 0.3 to 1 minute. The specific processingconditions are varied depending on the compositions of the metalsubstrate or metal fiber mat or woven metal fiber assembly used, itsthickness, and on the alloy to be formed on the surface. The layeredstructure of the finished catalyst consists essentially of an upperlayer of skeletal metals or alloys that has been optionally partlyabraded, an interdiffusion layer that has been optionally partlyexposed, and a residual substrate core (i.e., where the metal substratecore is usually the original metal of the metal substrate or theoriginal metal that made up the metal fibers in the metal fiber mat orwoven metal fiber assembly).

If coiled stock is desired, steps (b) through (g) optionally can beintegrated into a semi-continuous web process in which a continuouslymoving web of substrate material (i.e., here, the substrate material canbe a flat metal foil or a flattened metal fiber mat or a flattened wovenmetal fiber assembly) passes from one unit operation step to the next,or with optional intermediate recoiling, until all steps are completed.If honeycomb or similar structured scaffolding form factors are desired,forming and/or fastening steps preferably can be introduced after step(c) or after step (g), with subsequent processing conducted onindividual parts rather than in a web process.

A second aspect of the present invention is a method for making acatalyst support structure wherein an intrinsic oxidic support layer isdeliberately produced. The method comprises the following steps:

(a) preparing a slurry comprising one or more metal (includingprealloyed) powders including aluminum;

(b) coating a flat metal substrate or a flattened metal fiber mat or aflattened woven metal fiber assembly with said slurry;

(c) subjecting the coated metal substrate or coated metal fiber mat orcoated woven metal fiber assembly to heat under a reducing atmospherewhereby at least one of the one or more metal powders melts andinterdiffuses into the surface of the metal substrate or metal fiber mator woven metal fiber assembly;

(d) optionally leaching the coated metal substrate or coated metal fibermat or coated woven metal fiber assembly obtained in step (c) in acaustic solution;

(e) subjecting the coated and heat treated metal substrate, metal fibermat or woven metal fiber assembly obtained in step (c) or (d), ifpracticed, to heat in an oxygen containing atmosphere for an additionalperiod of time.

Subjecting the coated and heat treated metal substrate, metal fiber mator woven metal fiber assembly to heat for a second period of time in anoxygen containing atmosphere (i.e., step (e) above) can produce analumina or mixed metal oxide intrinsic catalyst support layer that ismore strongly adhered to the coated metal substrate, coated metal fibermat or coated woven metal fiber assembly and has a more uniformthickness distribution than a washcoat layer that is obtained from atraditional spray or dip coating process using a slurry mainly composedof metal oxides, applied to fully formed monolithic structural units.Catalytic species, such as precious or platinum group metals, can bedispersed on or into the intrinsic oxidic support layer using knownmethods. The temperature used in step (e) is from about 400 to 950° C.,preferably from about 640 to 850° C. The amount of time that the coatedmetal substrate, coated metal fiber mat or coated woven metal fiberassembly is held at those temperatures is from about 10 to 600 minutes,preferably from about 45 to 180 minutes.

In step (c) of the above-described process, the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly is heated toa temperature in the range of about 640-1100° C., preferably from about650-900° C., for a period of time from about 0.2 to 4 minutes,preferably from about 0.3 to 1 minute.

We contemplate that steps (b) through (e) optionally can be integratedinto a semi-continuous web process in which a continuously moving web ofsubstrate material (i.e., metal foil or flattened metal fiber mat orflattened woven metal fiber assembly) passes from one unit operationstep to the next, or with optional intermediate recoiling, until allsteps are completed. If honeycomb or similar structured scaffolding formfactors are desired, forming and/or fastening steps can be introducedmost advantageously after step (c), with subsequent processing conductedon individual parts rather than in a web process.

Yet another aspect of the present invention involves particularmacroporous multimetallic alloy or mixed metal formulations (i.e., onthe surface of a metal substrate) made during the disclosed processesfor making thin layer skeletal metal structured catalysts. The alloy ormixed metal formulations are particularly useful for conventional steammethane reformation to produce syngas from natural gas and steam, andfor hydrodeoxygenation of pyrolysis bio-oils, and yet inherentlyresistant to sintering. Other uses for the catalysts of the presentinvention include: (a) Fischer-Tropsch synthesis reactions (particularlyFe—Co—Zr—Al catalysts); (b) hydrogenations of fatty acids (particularlyover Ni—Zr—B—Al or Ni—Cr—B—Al catalysts); and (c) partial oxidations ofaromatics (particularly Au—Ni—Zr—Al catalysts).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of two embodiments of the present process to makea metal-supported catalyst structure.

FIGS. 2A, 2B and 2C are scanning electron microscope images showing acomparison of the surfaces of fresh (passivated) catalyst (2A), steamed(i.e., at 900° C. for 9 hours) catalyst (2B), and used catalyst that hadbeen on stream for several hundred hours (2C).

FIG. 3 is a graph showing the oxygen depletion in a light-off test ofn-heptane over a platinum group metal catalyst prepared on a corrugatedcatalyst support foil of the present invention.

FIGS. 4A, 4B and 4C are scanning electron microscope images of thetreated and untreated fibers used in Example 10.

FIG. 5 is composed of two pictures of samples of the plurality of thinsteel fibers that can be used to make the fibrous material of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be understood by those skilled in the art byreference to the following description of the preferred embodimentsincluding examples and the accompanying drawings.

In a first embodiment, this invention relates to a method for producingthin-layer skeletal catalyst-coated metal and metal alloy structureswithout applying an intermediate washcoat layer and, in a secondembodiment, to a method for generating an adherent oxidic layer on athin metal foil or fibrous substrate, suitable as a catalyst support,without applying an additional oxide slurry washcoat. Other aspects ofthis invention relate to the formation and use of specific catalyticallyactive skeletal metal layers whose compositions consist of particularsurface alloys or multiple metal mixtures produced by the method of thefirst embodiment.

FIG. 1 is a schematic of two embodiments of the process for producingthe skeletal catalyst-coated thin metal structures of the presentinvention. In these embodiments, a six inch wide roll of nickel or othermetal shim stock, for example, is coated by a slurry prepared by ballmilling a mixture containing one or more metal powders or preformed bulkmetal alloy powders that are precursors to catalytically activematerials. It should be noted that the slurry always contains aluminumpowder. The coated nickel roll stock is then introduced into a furnaceunder a reducing or inert atmosphere wherein at least one of the one ormore metals in the slurry interdiffuses with the surface of the nickelroll stock forming an alloy or intermetallic that firmly binds thecoating to the nickel substrate. Upon exiting the furnace, the coatedmetal substrate can be either formed into a desired shape or continuefor additional processing before being formed into a desired shape.

Although FIG. 1 and the following disclosure is predominantly directedto the use of thin metal foil as the metal substrate, similar processsteps would be used if the metal substrate was a flattened mat of metalfiber or a flattened woven metal fiber assembly.

In the embodiment where the coated metal substrate is a thin metal foilthat is formed into a desired shape before any additional processingoccurs, the coated metal stock is slit, optionally corrugated, chopped,and assembled upon leaving the furnace to form the desired catalyststructure. The desired catalyst structure may then be assembled into alarger structure, such as by arranging cut pieces and fastening them bymethods known in the art to produce a stackable scaffold superstructure.Then, the coated metal pieces are leached in an aqueous caustic solutionas a formed shape and subsequently subjected to a chelating acid bathsuch as a citric acid bath. Following the chelating acid bath, thecoated metal pieces are passivated in a bath of a mild oxidant,preferably dilute hydrogen peroxide aqueous solution, and thenwater-washed and dried. After passivation, but prior to the drying step,formed catalyst structures optionally can be abraded with a highvelocity jet of water to remove weakly adhered layers, if any, thendried. Finally, the formed catalyst structures can be packaged.

In another embodiment of the present invention, a conforming oxidiccatalyst support layer is formed on the surfaces of the adhered alloy orintermetallic layer (i.e., the layer formed after the one or more metalsin the coating slurry interdiffuses with the surface of the metal foilsubstrate). In this embodiment of the present invention, the intrinsicoxidic catalyst support layer is formed by adding an additional step tothe disclosed process. That step involves heating the coated metalsubstrate (after the leaching step or in place of the leaching step) inan oxygen containing atmosphere (e.g., air) to form an adherent oxidecoat (i.e., an intrinsic oxidic layer suitable as a catalyst supportlayer) directly, without application of an additional washcoat. Theadditional step preferably occurs in an air calcination unit after thegross form-factor fabrication step, if formed parts are desired. Thetopologies and specific surface areas of the oxidized surfaces aredistinctly different depending on whether or not the leaching step isapplied prior to the oxidation step. Thus, the need for inclusion of theleaching step is determined by the requirement for additional surfacearea or macroporosity in the finished catalyst support structure.

In the embodiment where the coated metal substrate is subjected toadditional processing before being formed into a desired shape, thecoated metal stock is subjected to a continuous leaching step in anaqueous caustic solution, before being subjected to a chelating acidbath. After the acid bath, the coated metal stock is subjected tocontinuous passivation by passage through a bath of dilute aqueoushydrogen peroxide solution for a time sufficient to quench anypyrophoricity. The coated metal stock may optionally be subjected tocontinuous abrasion using a high velocity jet of water (i.e., to causeabrasion of the outer surfaces) after being subjected to the acid bathand before or after the passivation step. After the passivation step,the coated metal stock is then rinsed with water and dried. Acetonerinsing can be used to accelerate drying, but care must be taken toprevent contact of acetone and hydrogen peroxide-containing solutions orresidues to avoid formation of potentially explosive compounds.Following the drying step, the coated metal stock is slit, corrugated,and chopped into the desired catalytic elements for the form factor ofinterest. The desired catalyst elements may then be assembled into alarger scaffold structure, such as by combining (e.g., use of stackingand fastening by methods known in the art) the smaller catalystelements. Finally, the formed catalytic scaffold structures can bepackaged.

In another embodiment of the present invention, a conforming oxidiccatalyst support layer is formed on the surfaces of the skeletal metalcatalyst layer (including within the macro pores and cracks in theskeletal metal catalyst layer). In this embodiment of the presentinvention, the intrinsic oxidic support layer is formed by heating thepassivated, leached metal substrate in an oxygen containing atmosphere(e.g., air) to form a thicker adherent oxide coat (i.e., an intrinsiccatalyst support layer). The additional step preferably occurs in an aircalcination unit after the scaffold gross form-factor fabrication step.

Regardless of whether the coated metal substrate is formed into adesirable shape before or after the additional processing steps, theleaching step is performed in a caustic solution, preferably a solutioncomprising NaOH. As is known in the art, the leaching step selectivelyremoves some of the aluminum and certain aluminides from the coating,forming porosity in the coating, but leaving in place various otheraluminide compounds. The temperature of the leaching bath is from about65 to 95° C., preferably from about 80 to 90° C. The amount of time thatthe coated metal substrate spends in the leaching bath is from about 5to 50 minutes, preferably from about 25 to 45 minutes.

The citric acid bath is only one embodiment of the present invention.Suitable acid baths would be those comprising, for example, a mineralacid or carboxylic acid, but polyprotic acids forming chelating anionsare preferred The temperature of the acid bath is from about 20 to 40°C., preferably from about 25 to 30° C. The amount of time that thecoated metal substrate is held in the acid bath is from about 2 to 10minutes, preferably from about 3 to 5 minutes.

The use of the intrinsic oxidic catalyst support layer is desirable whenvery low loadings of precious or platinum metal compounds are necessaryor desirable for catalysis or when complex molecular structures withbonded fragile ligands are required rather than bare zero-valent basemetals. Deposit of such materials directly onto a reactive base metalsubstrate, in the absence of an inert oxidic layer, could displaceligands or bury the precious or platinum metal catalytic top layer asthe metallic surface restructures with use. Furthermore, the chemicalselectivity of the composite metal surface catalyst would be altered bythe presence of the dominant reactive metal of the substrate. Thus, anintrinsic, catalytically inert oxidic catalyst support layer can actsimilarly to a ceramic washcoat layer known in the art to disperse suchcatalytic species without altering their site-specific activities.

Although lacking some of the advantages of a self-supported thin layerskeletal metal catalyst surface coating, the conforming oxidic catalystsupport layer is well adhered to the underlying substrate. Moreover, theintrinsic oxidic catalyst support layer is straightforwardly producedafter the gross-form fabrication step so as not to be damaged bymechanical processing. Once impregnated with active catalyst species,the uniform thickness of the intrinsic oxidic catalyst support layeralso promotes uniform temperature distribution to reduce the probabilityof damage from localized hot spots when structured catalysts are used topromote exothermic reactions such as the initiation of catalyticcombustion of organic vapors in an oxygen-rich gas.

In another embodiment of the present invention, catalyst alloycompositions have been created according to the disclosed methods thatare highly active and hydrothermally stable. The alloy compositions wereprepared using nickel foil substrates. The catalyst series investigatedon nickel substrates is based on compositions chosen using two-part,three-level partial factorial designed experiments that were pre-plannedto screen initial suitability for ternary alloys of nickel, aluminum,and a refractory metal. One of the refractory metals selected waszirconium. Pre-alloyed nickel-zirconium powders were used in some cases.The other refractory metals in the screening experiments were selectedbased on three criteria: having an elemental melting point significantlygreater than nickel, having the ability to form a ternary aluminide withnickel at a relatively low temperature, and having a relative bulk costlower than that of zirconium.

The other refractory metals (i.e., other than zirconium) selected werenickel (control), vanadium, chromium, titanium, tungsten, niobium,molybdenum, and tantalum. Replicates of each catalyst were prepared bycoating the nickel foil substrates with slurries that (when dried)contained the refractory metal at nominally 0, 5, and 11 weight percentloading (i.e., based on the weight of all of the metals in the appliedslurry) for screening purposes. After the furnacing step, allformulations were judged to be pliable and durable enough for mechanicalcorrugation and formation into honeycombs except several containingtitanium. Modified compositions of the titanium catalysts that containedslightly lower aluminum content were found to be acceptably pliable anddurable towards bending and corrugation directly after the furnacingstep. Screening of the metal supported catalyst containing the variousalloy compositions was performed to determine fitness-for-use. Screeningincluded analysis of: (1) relative activity in a low pressurehydrocarbon reforming reaction conducted in a quartz microreactor undera particular set of conditions; and (2) BET surface area retention aftersteam deactivation to measure extent of sintering. Table 1 shows theinitial conversion (i.e., percent conversion of total carbon atoms inthe reactant feed per total geometric area in cm² of the catalyst) formixed light hydrocarbon reforming (in a laboratory reactor) forcatalysts prepared according to the disclosed process.

TABLE 1 Initial Conversion per geometric area For Ni-only and CatalystCoating Compositions Ni alloy catalysts Ni-shim No Coating 0.57 Al—Ni—NiAl(59.4%)—Ni(39.6%)—B(1%) 0.87 only Ni—Zr-11%Al(52%)—Ni(36%)—Zr(11%)—B(1%) 1.50 Ni—Cr-11%Al(52%)—Ni(36%)—Cr(11%)—B(1%) 1.61 Ni—Ti-11%Al(52%)—Ni(36%)—Ti(11%)—B(1%) 1.54 Ni—V-11% Al(52%)—Ni(36%)—V(11%)—B(1%)1.27 Ni—Ta-11% Al(52%)—Ni(36%)—Ta(11%)—B(1%) 1.45 Ni—Zr-5%Al(56%)—Ni(38%)—Zr(5%)—B(1%) 1.63 Ni—Cr-5% Al(56%)—Ni(38%)—Cr(5%)—B(1%)1.41 Ni—Ti-5% Al(56%)—Ni(38%)—Ti(5%)—B(1%) 1.23 Ni—V-5%Al(56%)—Ni(38%)—V(5%)—B(1%) 1.38 Ni—Ta-5% Al(56%)—Ni(38%)—Ta(5%)—B(1%)1.24

Table 2 (below) shows the BET surface area before and after steamdeactivation. Because both initial activity and resistance todeactivation are important for optimal performance, Ni—Zr and Ni—Cralloys and higher loading of Ni—Ta are preferred compositions. Theremaining multimetallic alloy catalysts also perform better thannon-alloyed foraminous nickel compositions or compositions with addednickel plus aluminum only on the nickel substrate. The first entry inTable 2 was prepared by coating a nickel substrate with only aluminum asthe metallic component of the coating slurry. The last entry in Table 2was prepared by adding only aluminum, boron and nickel to the coatingslurry but no other metallic components.

TABLE 2 BET Surface Area Analysis of Multimetallic Alloy CatalystCoatings (not passivated) Surface Area Surface Area Before After Coatingcompositions Steaming¹ Steaming² Al(58.7%)—Ni (40.3%)—B(1%) 83 5Al(52%)—Ni(36%)—Zr(11%)—B(1%) 152 26 (RG-49-78)Al(52%)—Ni(36%)—Zr(11%)—B(1%) 153 29 Al(52%)—Ni(36%)—Cr(11%)—B(1%) 10021 Al(52%)—Ni(36%)—Ti(11%)—B(1%) 75 9 Al(52%)—Ni(36%)—V(11%)—B(1%) 15631 Al(52%)—Ni(36%)—Ta(11%)—B(1%) 132 25 Al(56%)—Ni(38%)—Zr(5%)—B(1%) 15827 Al(56%)—Ni(38%)—Cr(5%)—B(1%) 175 20 Al(56%)—Ni(38%)—V(5%)—B(1%) 15130 Al(56%)—Ni(38%)—Ta(5%)—B(1%) 146 24 Al(59.4%)—Ni(39.6%)—B(1%) 133 19¹Surface Area measured by BET (m²/g coating). All values are average ofvalues obtained for two batches of samples except for RG-49-78. ²SurfaceArea measured by BET (m²/g coating). All values are single values fromone batch.

Additional independent characterization experiments were conducted tofurther differentiate the multimetallic formulations of this inventionfrom those that did not include refractory alloy metals and from acommercial methane steam reforming catalyst comprising a nickel oxide ona ceramic support. Accordingly, passivated catalysts were reduced insitu prior to measuring their hydrogen chemisorption capacities by aflood adsorption/temperature programmed desorption method, a methodknown in the art. These data were compared to similar measurements onground and sized commercial nickel-based ceramic catalyst pellets(Hi-Fuel 110, Alfa-Aesar Johnson-Matthey Co.). Standard algorithms incommercial software that account for differences in metal loading wereused to estimate dispersions and metal areas derived from the hydrogenchemisorption data obtained. As shown in Table 3, replacing nickel witheither 11% (by weight) zirconium or chromium resulted in increasedhydrogen binding capacity, which translates to greater apparentdispersion, and in greater exposed metal area, directionally consistentwith earlier BET measurements. The specific surface areas anddispersions of nickel computed for fresh (passivated) multimetalliccatalysts of this invention are slightly higher than those areasmeasured for fresh Hi-Fuel-110 commercial catalyst after similarpre-reduction.

TABLE 3 Apparent Dispersion Measured by a Hydrogen FloodAdsorption/Temperature Programmed Desorption Method After a TemperatureProgrammed Reduction Step Average Apparent % Dispersion after TPRComposition of Original (assume 0.5 stoichiometry Catalyst SampleCoating for all coated metal content) Hi-Fuel-110 (Alfa-Aesar) Unknown7.8 ZrNi passivated Al(52%)—Ni(36%)—Zr(11%)—B(1%) 12.0 ZrNi steamedAl(52%)—Ni(36%)—Zr(11%)—B(1%) 2.5 CrNi passivatedAl(52%)—Ni(36%)—Cr(11%)—B(1%) 10.3 CrNi steamedAl(52%)—Ni(36%)—Cr(11%)—B(1%) 1.7 Ni only passivatedAl(59.4%)—Ni(39.6%)—B(1%) 8.4 Ni only steamed Al(59.4%)—Ni(39.6%)—B(1%)2.3

EXAMPLE 1

An 11% Zr—Ni multimetallic catalyst (designated as RG-49-78) wasprepared as described below. First, a 900 mL slurry containing 611 g ofAl powder (10 micron average particle size), 109.1 g of Ni powder(Conductive Nickel Pigment type 525 D, −250 mesh obtained from Novamet),444.3 g of Zr/Ni alloy (30/70; obtained from Chemetall), 10.1 g boronpowder (elemental amorphous boron 95%; 0.5 to 3 microns; obtained fromCR supply), 20.9 g of methyl methacrylate based binder, and 397.1 g ofacetone were milled in a ball mill for about 12 hours with 150 mL of ¼inch steel balls. The resultant slurry was then applied by dip coatingto a Ni shim stock substrate (nominally 2 mil thick) that had beenpre-cleaned with acetone and the resulting slurry coating thicknessranged from 5.75 to 6.25 mil. After drying in a hot air stream, thecoated substrate was passed through a four foot long furnace fitted withan open-ended retort at a speed of 12 ft/min. The temperature in thefurnace was 750° C. and the atmosphere was hydrogen gas (the hydrogengas flow rate to the furnace was 200 SCFH). After exiting the furnace,the coated substrate was leached in a 200-225° F. aqueous solutioncontaining 25% NaOH for 45 minutes and then rinsed with water. Thecoated substrate was then bathed in a citric acid solution (5% by weightcitric acid in water) for 3 minutes and then again rinsed with water.The coated substrate was then passivated by immersion in an aqueoussolution containing 3% (by weight) H₂O₂ for 12 minutes and subsequentlyrinsed with water again. Finally, the coated substrate was rinsed withacetone and dried with nitrogen gas.

The catalyst demonstrated an initial (passivated) surface area of 151m²/(g-coating), and aliquots were subjected to successive steamingperiods or to successive periods of attrition using a high pressurewater jet. Durability was judged by monitoring weight loss versusattrition time (see Table 4 below) and versus jet pressure and bymonitoring SEM thickness measurement after cross sectional polishing(see Table 5 below). Weight loss and surface area measurement were alsomade after successive periods of steaming at 900° C.

TABLE 4 Weight Losses of a Zr—Ni Catalyst After Successive Periods ofSteaming or Water Jet Abrasion High Pressure Water Steaming of JetRG-49-78 (about 1880 RG-49-78 at 900° C. psi at max nozzle setting)Treatment Treatment Time (hr) % Weight Loss Time (sec) % Weight Loss 24.04 30 9.93 3 5.36 150 10.27 6 3.13 300 8.74 9 3.76 — —

TABLE 5 SEM Coating Thickness Measurements of Distinct Layers in Fresh,Steam-Aged and Used Catalysts Inner Coating Total Coating (one side-bySample (By SEM) (μm) SEM) (μm) RG-49-78  7.5 ± 1.6 45.8 ± 7.3 FreshRG-49-78  7.4 ± 1.3 28.1 ± 7.0 Steamed at 900° C. for 9 hours RG-49-7810.1 ± 2.0 33.6 ± 6.8 Used

Successive abrasion of the catalyst resulted in an initial weight lossof the friable material that leveled out after an 8-9% by weight loss.Steaming for various periods up to 9 hours results in smaller weightlosses that did not continue to increase after the initial loss.Scanning electron microscope (SEM) measurement of the polished crosssections strongly supports the conclusion that initial attrition is dueto loss of a relatively loosely bound outer surface layer, but that noor little loss occurs in the highly active inner core that appears to bethe diffusion layer, which is firmly affixed to the metal substrate.

Table 6 (below) shows the relative initial hydrocarbon reformingactivity (i.e., percent conversion of total carbon atoms in the reactantfeed per total geometric area in cm² of the catalyst) of the samecatalyst type (identified as RG-49-78) after attrition and shows BETsurface areas before and after steaming. The inner core, exposed afterattrition, is just as or more active than the fresh catalyst withinexperimental error. Thus, the catalyst is shown to remain stable andextremely active after an initial loss of outer layer. Moreover,deactivation does not progress linearly with continuing time of exposureto highly abrasive conditions or to conditions that facilitate rapidsintering. Further, no detectable phase change (determined from DTA/TGAdata) exists at high temperature after the initial reduction of thepassivation oxide. Reduction of the oxide accounts for a large portionof the weight loss observed on stream. FIGS. 2A, 2B and 2C show theeffectiveness of steaming at simulating the sintering encountered duringaging of the catalyst on-stream as evidenced by the loss of sharp edgesof crystallites and formation of spherical structures in similarfashion.

Also, as shown in Table 6, the BET surface areas of the resultingmaterials are similar after steaming and after extended exposure toprocess conditions.

TABLE 6 Initial Light-Hydrocarbon Reforming Activity After IncreasingPeriods of Catalyst Attrition, and BET Surface Areas After IncreasingPeriods of Steaming Compared to Used Catalyst BET SA Passivated- BET SAPassivated Steamed Sample Treatment Time Xc/geometric area (m²/gcoating) (m²/g coating) RG-49-78  0 sec 1.40 — — (previous batch)Reference RG-49-78  30 sec 1.21 — — (pressure washed) RG-49-78 300 sec1.52 — — (pressure washed) RG-49-78 2 hr — 147 16 (previous batch)Reference RG-49-78 6 hr — — 14 (steamed at 900° C.) RG-49-78 9 hr — — 15(steamed at 900° C.) RG-49-78 — — — 14 Used

EXAMPLE 2

An experiment was run where an 11% Zr/Ni/Al/B/Ni catalyst was preparedfor use in a steam methane reforming reaction, according to anembodiment of the methods disclosed in the present invention. First, a900 mL slurry containing 611.0 g of Al powder (10 micron averageparticle size), 109.1 g of Ni powder (Conductive Nickel Pigment type 525D, −250 mesh obtained from Novamet), 444.3 g of Zr/Ni alloy (30/70;obtained from Chemetall), 10.1 g of boron powder (elemental amorphousboron 95%; 0.5 to 3 microns; obtained from CR supply), 20.9 g of methylmethacrylate based binder, and 397.1 g of acetone were milled in a ballmill for about 12 hours with 150 mL of ¼ inch steel balls. The resultantslurry was then applied by dip coating to a Ni shim stock substrate(nominally 2 mil thick) that had been pre-cleaned with acetone and theresulting slurry coating thickness ranged from 5.75 to 6.25 mil. Afterdrying in a hot air stream, the coated substrate was passed through afour foot long furnace fitted with an open-ended retort at a speed of 12ft/min. The temperature in the furnace was 900° C. and the atmospherewas hydrogen gas (the hydrogen gas flow rate to the retort was 200SCFH). After exiting the furnace, the coated substrate was leached in a200-225° F. aqueous solution containing 25% NaOH for 45 minutes and thenrinsed with water. The coated substrate was then bathed in a citric acidsolution (5% by weight citric acid in water) for 3 minutes and thenagain rinsed with water. The coated substrate was then passivated byimmersion in an aqueous solution containing 3% (by weight) H₂O₂ for 12minutes and subsequently rinsed with water again. After the H₂O₂treatment, the coated substrate was then abraded with a high velocityjet of water at 1880 psi for 2 minutes on each side with the coatedsubstrate fixed at 3 inches distance from the nozzle of the water jet.The water jet abrasion caused the coated substrate to lose approximately3.9% of its mass. Finally, the coated substrate was rinsed with acetoneand dried with nitrogen gas and then passivated by immersion in anaqueous solution containing 3% (by weight) H₂O₂ for 12 minutes andsubsequently rinsed with water again then dried in air.

EXAMPLE 3

Scanning electron microscope images (SEM) were taken of fresh, steamedand used catalytic foils derived from the same preparative batchdescribed in Example 2. The used samples had been exposed to SMR processconditions for several hundred hours prior to imaging. In the images,the outer layer appears less prominent and highly diminished inthickness with aging. The inner core however remains adhered and highlyreactive either after the steam treatment or after being used underprocess conditions for several hundred hours.

EXAMPLE 4

A sample of zirconium-nickel foil composition, prepared according to themethod described in Example 2, that had been furnaced at 900° C. underhydrogen, anaerobically leached, citric acid treated, passivated andpre-attrited using a high pressure water jet, and then dried, was testedfor steam methane reforming performance. In a packed-bed plug-flowintegral conversion reactor, operating under commercially viable steammethane reforming conditions, instantaneous on-line analyticalmeasurements of product composition and corresponding temperaturedifferentials between applied temperature at the reactor wall and thecatalyst surface were used to compute activity versus time. Bulk heatand mass transfer effects on activity that could be attributable to formfactor differences were excluded in these tests by using similarly sizedfoil slivers embedded in inert packing rather than using monolithic formfactors. However, intraparticle (molecular scale) mass transfer effects,dependent on the surface nanostructures of the foils, still couldinfluence relative performance.

In this test, the catalyst foil not only generated higher volumetricactivity than the reference catalyst, a ceramic-supported nickel oxideof a type used commercially, but also survived 900 hours withoutdeactivation or significant physical degradation.

The following examples demonstrate the effectiveness of the coated foilsor fibers of the present invention to act as catalyst supports forpromoted precious or platinum group metal catalysts. The substrate metalused to produce these catalysts was 430 grade stainless steel (awell-known alloy that contains no significant amounts of aluminum oryttrium). The 430 grade stainless steel alloy is a preferred metalsubstrate material for the catalysts of the present invention,especially when those catalysts need to be resistant to hightemperatures (e.g., temperatures in the range of 800 to 1,000° C.).

EXAMPLE 5 Preparation of Catalyst Supports

430 alloy grade stainless steel foil shim stock, 2 mil thick, wasobtained from Ulbrich Co. of North Haven, Conn., USA as 4 inch widepieces. These were cut into flat strips measuring approximately 1.5 inchby 8 inch and washed with acetone. Each of several strips was coatedwith a slurry using a laboratory-scale falling-film “dip coater” thendried by hanging in a heated air stream. The coating slurry was composedas follows:

aluminum powder (about 3 μm average particle size): 57.8 wt %

methyl acrylate based binder: 4.2 wt %

Acetone: balance

After drying, the coated strips were stapled to leaders of metal foiland passed through a 4 foot long retort housed in a clam shell furnaceheld at 730° C. at 6 feet per minute under flowing hydrogen, then cooledin air. The coating appeared uniform and adherent at this point. The“hydrogen furnaced” intermediates were placed in a box furnace on aceramic fixture that allowed air circulation on both sides of the foilstrips and were heated in static air with the following schedule: roomtemperature to 650° C. ramped at 20°/min then held at 650° C. for 1.5hours. The samples were allowed to cool in the box furnace over about1.5 additional hours. The resulting oxidized strips were flexible, withthe coating remaining intact after flexure.

EXAMPLE 6 Catalyst Preparation A: Ce/Cu/Pd/Pt Catalyst

Support strips that were prepared as described in Example 5 were cutinto smaller pieces of 25 by 80 mm dimensions and impregnated withcatalytic agents as described below. Ammonium hexanitrocerate (IV),(NH₄)₂Ce(NO₃)₆ (0.3958 g), and copper (II) acetate hydrate (0.0262 g)were dissolved into 0.5 mL of distilled water with sonication, thenacetonitrile (0.11 g) was added to form a lime green solution. Thesupport strip was placed on a watch glass and wet with the Ce—Cusolution on both sides. Excess solution was tapped off the metal stripback onto the watch glass, then the wet strip was dried for severalminutes in a hot air stream. After drying, the sequence of impregnationand drying was repeated two more times until the solution had beendepleted. The dried strip was calcined in a static box furnace inambient air and heated by ramping at 20° C./minute to 500° C., then heldat this temperature for 30 minutes and then cooled. The cooled strip waslightly wiped with a laboratory tissue then blown clean with acompressed air nozzle to remove a small quantity of loose powder fromthe surface. The strip then was impregnated with palladium and platinumsalts as described below. Dichlorotetraaminepalladium (II) monohydrate,Pd(NH₃)₄ (Cl)₂. H₂O (0.0766 g) and tetraamineplatinum (II) nitrate,Pt(NH₃)₄ (NO₃)₂ (0.0367 g) were dissolved into 0.5 mL of distilled waterwithout pH adjustment. The strip was wet with the Pd/Pt solution asdescribed above, then the strip was dried in a hot air stream. Thewetting/drying sequence was repeated carefully until the entire quantityof solution had been consumed. The dried strip was calcined in a boxfurnace in air by heating at 20° C./min to 500° C., then held at 500° C.for 2 hours and then cooled slowly in the furnace. The sample waslightly wiped with laboratory tissue then blown clean with compressedair. Weight measurement showed that the sample had gained weight in thecoating-calcining-wiping process. Based on the assumptions of retentionof applied molar ratios, the composition of expected reaction productphases, and the actual final weight gain, the nominal catalystcomposition was computed as 5.56% CeO₂, 0.51% CuO, 0.828% Pt, and 1.59%PdO.

EXAMPLE 7 Catalyst Preparation B: Ce/Pd/Pt Catalyst

A second catalyst was prepared as above (i.e., as described in Examples5 and 6), except the copper salt and acetonitrile components wereexcluded from the formulation, only one calcination step was included,and the following quantities of precursor materials were used for asimilarly sized support strip:

(NH₄)₂Ce(NO₃)₆ 0.380 g Pt(NH₃)₄ (NO₃)₂ 0.030 g Pd(NH₃)₄ (Cl)₂•H₂O 0.075g

EXAMPLE 8 Catalyst Preparation C: Pd/Pt Catalyst

A third catalyst was prepared as above (i.e., as described in Examples5, 6 and 7), except both the cerium and copper salts and theacetonitrile components were excluded from the formulation, only onecalcination step was included, and the following quantities of precursormaterials were used for a similarly sized support strip:

Pt(NH₃)₄ (NO₃)₂ 0.028 g Pd(NH₃)₄ (Cl)₂•H₂O 0.074 gCatalyst Testing for Oxygen Depletion

Each of the three catalysts described above (i.e., A, B and C), weretested in a laboratory scale oxygen depletion reactor screening test todetermine their relative “light off” temperature performance. The testis conducted by passing a precise flow rate of a particular gaseousreactant solution containing a hydrocarbon (heptane) and oxygen over afixed size of coiled, corrugated catalytic foil. Oxygen is the limitingreagent. As the temperature is ramped upwards at a precisely controlledrate, the product stream is continually analyzed for residual oxygencontent. The isokinetic temperature (i.e., corrected for the time delayof transport between the reactor and the analyzer) at which point 50 mol% conversion of oxygen has occurred, termed T₅₀, is noted and used as acomparative measure of “light off” performance. Other features ofimportance are the initiation temperature and the shape and slope of theextinction curves (mathematically related to catalytic reaction rate)relative to those observed over other catalysts. Reaction rate data arenormalized to reactor bed volume and to total exposed geometric surfacearea of catalytic foil. The test conditions used are described in Table7 (below).

TABLE 7 Catalyst size/form coiled, corrugated foil of 25 by 80 mmdimensions before corrugation Catalyst geometric area 40 cm² Feedcomposition O₂: 1.1%; n-C₇H₁₆: 21.15%; Ar: 27.19% (internal standard);N₂: balance T ramp 7° C./min; 25 to 370° C. (typical) GHSV¹ (based ontotal reactor vol) ~8400 h⁻¹ Hourly Face Velocity (based on actual 275.9cm/h area on both sides of catalyst) Reaction overall stoichiometryC₇H₁₆ + 11 O₂ → 7 CO₂ + 8 H₂O Static pressure Barometric Analyticalquadrupole mass spectral analysis of O₂ partial pressure at m/e = 32.0¹The GHSV (gas-hourly-space-velocity) was calculated at 25° C. and 760torr. The reactor volume is computed as if it were a fully filledcylinder with no void volume between portions of catalyst.

Catalysts often equilibrate with repeated use as the Pt/Pd species onthe surface adjust oxidation state ratios (reduce or oxidize) to achievea steady-state condition. In this test, changes occur over 2-3 runsduring which time their T₅₀ values typically move to lower values andtheir curve shapes become more symmetrical and often steeper inextinction of oxygen (catalysts get better). When time permits, thethird run over each catalyst usually is taken as the definitive resultfor that particular composition in our laboratory.

Table 8 (below) and FIG. 3 summarize the corrected data for thecatalysts prepared as described above in Examples 6, 7, and 8. In FIG.3, the y-axis values represent the O₂ partial pressure (in torr)measured by a mass spectrometer (i.e., determined from the O₂ signal atm/e=32). The x-axis values are the corrected catalyst temperature (i.e.,isokinetic temperature; approximately two degrees celsius lower thanmeasured temperature at catalyst bed) in degrees celsius”.

Note that addition of cerium lowers the temperature at which thecatalyst initiates the “light off” but does not affect the T₅₀ much. Theco-addition of a copper component in addition to the cerium promotes aneven lower temperature for initiation of the reaction and dramaticallyreduces T₅₀ as well. Table 8 (below) summarizes the quantitative data:

TABLE 8 Example Run T₅₀ (° C.) (lower is better) 6 - CatalystPreparation A 1 209 [Ce/Cu/Pd/Pt] 7 - Catalyst Preparation B 3 245[Ce/Pd/Pt] 8 - Catalyst Preparation C 3 247 [Pd/Pt]Fibrous Material Embodiment

In a preferred embodiment of the present invention, thin metal fibers,such as thin 430 stainless steel fibers, are used as the startingmaterial upon which aluminide and aluminum oxide layers are formed tocreate a fibrous material that functions as an effective catalystsupport for platinum group or other metals. After impregnation withcatalytic agents, this catalytic fibrous material (which can withstandhigh temperatures, for example from 600-900° C.) can be used in soottraps for diesel engines or in catalytic converters. For example, thecatalytic fibrous material (e.g., containing a PGM catalyst) can be usedto create a self-oxidizing partial diesel particulate filter (i.e., apartial diesel particulate filter conforms to the requirement for soottrapping efficiency of greater than 50% but less than 80%) featuringpartial bypass of flow to ensure even loading of soot and to reducepressure drop.

Catalytic soot traps filter carbonaceous soot particles from exhauststreams under rich combustion conditions and ignite and burn off thesoot under lean combustion conditions via catalytic initiation ofoxidation.

Most catalytic soot trap devices comprise a refractory oxide particlelayer that is applied directly to a ceramic or metallic substrate beforethe catalytic species is applied. The refractory oxide particle layer iscommonly referred to as a “washcoat,” and its application is followed byapplication of the PGM components. The washcoat typically comprises highsurface area oxides, such as, for example, transition-phase aluminumoxides, that stabilize and disperse the catalytic components (i.e., theymaintain the specific surface area of the catalytic components underprocess conditions, without becoming volatile or detrimental to thecatalytic species). The PGM, or other catalytic components, can then beapplied in a second impregnation step over the cured washcoat layer, orcan be added directly to the slurry of the washcoat particles (andadsorbed thereon) prior to coating of the metallic or ceramic substrateforms in a single coating step. The washcoat components may alsocomprise hydroxide precursors that cross-link during curing to formoligomers characterized by bridging oxygen groups. While thecross-linking improves adherence to the substrate, the washcoat is notdirectly covalently bonded to the metallic substrate.

Regardless of the specific composition, high solids content viscousslurries of washcoat materials are difficult to apply in uniformthickness over small formed openings or open channels that are part ofthe pre-shaped substrate structures. When metal supported catalysts areused to catalyze exothermic processes such as combustion, hot spots canarise if the washcoat, and the catalytic layer supported thereon, is notapplied uniformly. Hot spots lead to accelerated failure of substratematerials as well as to deactivation of the catalytic components. Thus,uniform coating of substrate structures facilitates the production ofdurable catalytic devices.

In addition to challenges associated with application of the washcoatlayer, the constituent materials used to prepare substrates (monoliths),whether ceramic or metallic, must be able to withstand severe processingconditions. These constituent materials must also be suitable forfabrication into form-factors that can trap soot without too high apressure drop. The materials used to prepare substrates must also becompatible with the chosen washcoat layer to avoid delamination of thislayer. Heat conduction, cost and corrosion-resistance in the presence ofacidic gases such as sulfur oxides and steam are also important factorsto consider when choosing substrate materials.

Metallic substrates are sometimes preferred over ceramic substratesbecause they can more easily be formed into complex shapes and/orbecause they are better at heat conduction. On the other hand, ceramicsare sometimes preferred over metallic substrates due to the fact thatmany metals are easily corroded or destructively oxidized under severeprocess conditions. High alloy metals that are corrosion and oxidationresistant do find use as catalyst supports, but they are limited to highvalue added applications because of their high cost.

When metallic substrates are used, practitioners often employ hightemperature stable alloys such as Fecralloy™. Due to its composition,Fecralloy™ generates, upon exposure to air, a thin aluminum oxide layeron the external surface of the metallic substrate that is protectiveagainst corrosion and oxidative degradation. The protective layerdiffers from washcoat oxide layers—which are designed to be porous—inthat it is thinner and less pervious.

Unfortunately, fibrous forms of Fecralloy™ are expensive and difficultto work with. For example, some metal oxide washcoats adhere poorly tothis alloy when applied by the usual slurry process without priortreatment of the surface. Many other metals such as corrosion-resistanthigh alloy steels can be used for high temperature applications, butthese are extremely expensive or are too brittle to easily fabricateinto coiled structures of appropriate fiber density needed for efficientsmall particle filtration.

Based on the aforementioned challenges, it would be desirable to providea metallic material (and process for its production) for use as acatalyst substrate that exhibits good heat conduction, is corrosionresistant, is oxidation resistant, and provides for a uniformlydistributed, and highly adherent, oxide layer. Such material should alsobe capable of being formed into a fibrous form factor (e.g., a monolithhoneycomb; or a packed fiber body; or parallel plates) that is suitablefor coating with platinum group metal catalytic components and that issuitable for fabrication into soot traps for diesel engines.

The catalytic soot trap embodiment of the present invention comprises afibrous material comprising a highly adherent aluminum oxide outer layerof substantially uniform thickness. The aluminum oxide outer layer ofthe fibrous material acts as an intrinsic support for catalyticmoieties, such as platinum group metal (PGM) catalytic moieties. Thefibrous material of the instant invention is malleable enough to befabricated into a low pressure-drop form factor, exhibits good heatconduction, is corrosion resistant and is resistant to oxidation. Inaddition to being used as a soot trap for diesel engines, the fibrousmaterial (after the inclusion of the PGM) can also be used in catalyticconverters.

In one aspect of the catalytic soot trap embodiment of the presentinvention, the fibrous material comprises a plurality of thin steelfibers comprising a chromium-iron alloy, an interdiffusion layer (alsoreferred to herein as an aluminide layer) covering at least a portion ofthe surfaces of the plurality of thin steel fibers, and an aluminumoxide outer layer that adheres to, and interfaces with, theinterdiffusion layer. The interdiffusion layer (also referred to hereinas the aluminide layer) may comprise aluminum, substrate metal (heresteel) and aluminides (i.e., intermetallics of aluminum and thesubstrate metal or metals contained in the substrate metal). In apreferred embodiment of the present invention, the aluminide layercovers most of the surface of the plurality of thin steel fibers and thealuminum oxide outer layer covers most of the surface of the aluminidelayer.

The aluminum oxide outer layer has a substantially uniform thickness. Inaddition, this layer is sufficiently thick enough to act as a suitablesupport layer for catalytic moieties such as platinum group metalcatalytic moieties and also provides protection against oxidation andcorrosion. In a preferred embodiment of the present invention, thealuminum oxide outer layer is highly adherent to the aluminide layer.

The plurality of thin steel fibers is usually in the form of a pliablemetal fiber bundle and comprises Type 430 or Type 434 steel.Furthermore, the plurality of thin steel fibers are typically able toretain ductility after being heated in air to temperatures of about 600°Celsius to about 900° Celsius.

The fibrous material of the present invention can be made by a methodcomprising the following steps:

-   -   (a) obtaining a plurality of thin steel fibers comprising a        chromium-iron alloy;    -   (b) applying an aluminum coating onto the plurality of thin        steel fibers;    -   (c) forming an aluminide layer by partially interdiffusing the        aluminum from the aluminum coating into the surface of the        plurality of thin steel fibers by heating in a diffusion furnace        under an inert or reducing atmosphere; and    -   (d) forming an aluminum oxide outer layer (i.e., on the surface        of the aluminide layer) having a substantially uniform thickness        by subjecting the residual surface aluminum and aluminide layers        to high temperature oxidation (e.g., in an oxidation furnace) in        the presence of an oxidizing gas such as air.

The aluminum coating is typically formed from a slurry that comprises analuminum powder and a binder. The aluminum coating can also comprise astabilizer such as cerium oxide or other rare earth salts. The aluminumcoating is usually applied to the plurality of thin steel fibers using acontinuous falling film coater followed by a drying step.

The interdiffusion of the aluminum from the aluminum coating into thesurface of the metal fibers takes place in a diffusion furnace at atemperature of from about 640 to 1,100° C., preferably from about 650 to900° C., under a hydrogen atmosphere for a time of from about 0.2 to 4minutes, preferably from about 0.3 to 1 minute.

The plurality of thin steel fibers containing an aluminide coating isusually heated in the oxidation furnace to a temperature of about 600°Celsius to about 950° Celsius for about 5 to about 120 minutes. In apreferred embodiment of the present invention, the aluminide coatedfibers are heated in the oxidation furnace to a temperature of about800° Celsius to about 950° Celsius for about 10 to about 15 minutes.

The fibrous material of the present invention can be made into a filter(e.g., a catalytic filter) by a method which includes performing steps(a) through (d) above and then steps (e) and (f) as described below:

-   -   (e) impregnating the plurality of thin steel fibers bearing the        aluminide and aluminum oxide layers with a platinum group metal        catalyst so as to infiltrate at least a portion of the aluminum        oxide layer with the platinum group metal catalyst; and    -   (f) forming a filter from the fibrous material obtained from        step (e).

Another method of making a filter according to the present inventioncomprises performing steps (a) to (d) as described above and then steps(e) and (f) as described below:

-   -   (e) forming a filter from the fibrous material obtained in step        (d); and    -   (f) impregnating the filter obtained in step (e) with a platinum        group metal catalyst so as to infiltrate at least a portion of        the aluminum oxide layer with the platinum group metal catalyst,        optionally followed by calcination.

The fibrous material comprising the aluminum oxide outer layer does notdisintegrate (e.g., no delamination of the aluminum oxide coating) whensubjected to substantial temperature changes, exhibits good heatconduction, is oxidatively stable (as measured by thermogravimetricanalyzer (TGA), differential scanning calorimetry (DSC), and/or longtemperature soaks in a heated air environment), and is not excessivelybrittle (i.e., it is malleable enough to be fabricated into a lowpressure-drop form factor).

Experiments performed to measure the oxidative stability, measured by athermogravimetric analysis (i.e., TGA), of two samples of fibrousmaterials showed that one of the samples (Sample 2), which was a coatedfibrous material according to the instant disclosure, was moreoxidatively stable in air at temperatures exceeding 1,000° Celsius thanthe other sample (i.e., Sample 1), which was an uncoated stainless steelfiber mat. Specifically, the two samples were tested for oxidativestability by heating the samples in flowing air (50 mL/min) from roomtemperature to 1,200° C. at a ramp rate of 20° C. per minute. In thisoxidative stability experiment, the sample according to the presentinvention (i.e., Sample 2; a mat of thin steel fibers bearing a uniformaluminum oxide coating that is adhered to and interfaces with analuminide layer) showed a weight gain just 1.4%, whereas the othersample (i.e., Sample 1; a mat of uncoated stainless steel, alloy 434,fibers) showed a weight gain of 9.2% under the same oxidationconditions. Sample 2 was made by the process described below.

A ¼ inch thick×2 inch wide 430 stainless steel fibrous mat (obtainedfrom Ribbon Technology Corporation) was cut into strips measuringapproximately 2 inch by 8 inch and those strips were cleaned withacetone. Each of the strips was coated with a slurry using a laboratoryscale dip coater and then dried by hanging in a heated air stream. Thecoating slurry was composed of: (a) 58% by weight aluminum powder (about3 micron average particle size); (b) 4% by weight methyl methacrylate(as a binder) and (c) 38% by weight acetone. After the drying step, thecoated strips were stapled to leaders of metal foil and passed through(at six feet per minute) a four foot long retort housed in a clamfurnace held at 710° C. The atmosphere in the furnace was flowinghydrogen. After emerging from the furnace, the samples were cooled inthe ambient air. The hydrogen furnaced samples were then oxidized usinga batch process wherein the samples were placed in a box furnace andheated in static air using the following oxidation conditions: roomtemperature to 630° C. ramped at about 5° C. per minute and then held at630° C. for two hours before the temperature was ramped from 630° to850° C. at about 2.4° C. per minute and held at 850° C. for two hours.The samples were then cooled to room temperature in ambient air. Theresultant samples were flexible.

In a preferred embodiment of the present invention, the fibrous materialof the instant disclosure is made by first obtaining coarse, medium, orfine grade 430 or 434 chromium-iron alloy stainless steel coiled mats ofa suitable width and thickness, wherein each mat comprises a pluralityof bundled thin steel fibers. FIG. 5 depicts two samples (ASC54-18-1 andASC54-18-2) of the plurality of thin steel fibers (shaped into loosemats) that can be used to fabricate the fibrous material of the instantdisclosure. The dimensions of these samples are shown below in Table 9.

TABLE 9 Length Width Height Sample ID (inches) (inches) (inches) Volume(in³) A5C54-18-1 1.75 2.25 0.25 0.98 A5C54-18-2 2.75 1.75 0.25 1.20

In addition, FIG. 4A shows an SEM image of an individual thin steelfiber (i.e., untreated; as received) used to make the fibrous materialof the present disclosure. The plurality of thin steel fibers comprisesa chromium-iron alloy stainless steel that usually comprises about 14%to about 18% chromium. In addition, the mat of fibers is typically about⅛ to about ½ inches thick and about 2 to about 4 inches wide, althoughwider mats of fibers can be used if desired.

Next, the plurality of thin steel fibers are coated with an aluminumcoating. The aluminum coating is typically in the form of a slurry thatcomprises an aluminum powder, a binder (e.g., a methyl acrylate-typebinder) and a solvent (e.g., acetone). The slurry may also comprise astabilizing component such as cerium oxide or other rare earth salts.Usually, the slurry is ball milled with stainless steel balls overnightand is continuously stirred during the coating process.

Coating of the plurality of thin steel fibers with the aluminum coatingis typically accomplished using the falling film method. The speed atwhich the plurality of thin steel fibers rise, and the length of thezone before the dryer, can both be adjusted according to skills known inthe art to generate a substantially uniform coating of a desirablethickness.

After the aluminum coating has been applied, the plurality of thin steelfibers move vertically through a drying section to set the binder. Oncethe binder is set, the plurality of thin steel fibers are subjected to adiffusion furnace heated to a temperature of about 700 to about 760° C.for about 0.5 to about 4 minutes while continuously moving through aretort under a flowing hydrogen atmosphere. The steel fibers then emergeinto air and are cooled for a sufficient time to bring them to roomtemperature. The diffusion furnacing allows the aluminum coating topartially diffuse into the plurality of steel fibers and form analuminide layer. Without wishing to be bound by any theory, thesepartially diffused aluminide layers may contribute to corrosionresistance and adherence of the aluminum oxide outer layer formed duringthe oxidative furnacing step.

The method of the present embodiment of the instant invention does notrequire a liquid bath leaching step after reductive furnacing (i.e., thediffusion furnacing step) is completed. Therefore, once the steel fibermats have been cooled, they are subjected to forming steps followed byoxidative furnacing or subjected directly to oxidative furnacing priorto any forming. The steel fiber mats are typically oxidatively furnacedfor approximately 5 to 15 minutes. In addition, the oxidative furnaceoperates under flowing air and is usually heated to a temperature ofabout 600° Celsius to about 950° Celsius. More typically, the oxidativefurnace is heated to a maximum temperature of about 650° Celsius toabout 850° Celsius. If the fiber mat is formed into a shaped body afterdiffusion furnacing and prior to oxidation furnacing, the fully shapedbody (i.e., in finished form) is placed into a static furnace foroxidative treatment in air.

The oxidative furnacing produces a substantially uniform aluminum oxideouter layer on the steel fibers by oxidizing the residual aluminum lefton the surface of the plurality of steel fibers after diffusionfurnacing. The resulting aluminum oxide outer layer interfaces with (andis strongly adhered to) the aluminide layer. The interaction between thealuminum oxide outer layer and aluminide layer provides high temperaturestability to the fibrous material. FIGS. 4B and 4C show SEM images ofone of the thin steel fibers of the instant disclosure after thealuminum oxide outer layer has been formed.

In addition, the aluminum oxide outer layer is thick enough to act as acatalyst support with or without a washcoat layer. The thickness of thisaluminum oxide outer layer can be controlled, by some degree, byadjusting the thickness of the initial aluminum coating, by thediffusion furnacing conditions, and by the degree of oxidation of thealuminum. Furthermore, the resulting surface area of the aluminum oxideouter layer can be controlled by adjusting the final calcinationtemperature or by adding stabilizing components (e.g., cerium oxide) ina small quantity, either to the coating slurry or introduced byimpregnation as aqueous cerium salt solutions after the aluminum oxidecoating is formed, followed by calcination.

The catalytic soot trap embodiment of the present invention is furtherillustrated in the non-limiting examples described below.

EXAMPLE 9

A ¼ inch thick×2 inch wide coarse, medium, or fine grade 430 or 434coiled stainless steel mat is fed into a continuous falling film coater(no Mayer rods) and the mat is coated with an aluminicious slurrycomprising 62% solids (aluminum powder; about 10 micron average particlesize) and 10% methylacrylate-type binder in acetone, wherein the slurryhas been previously ball milled with steel shot overnight. Cerium oxideis optionally added at a dosage of about 2% of the anticipated aluminumoxide. The slurry is continuously stirred during the coating process.

The coated steel fiber mat rises vertically at about six feet per minuteto generate a substantially uniform coating by the falling film method.The web speed and vertical distance over which the falling film drops(i.e., the length of the zone before the dryer) can be optimized toadjust the thickness of the aluminicious slurry coating. After coating,the coated steel fiber mat moves vertically through a drying section toset the binder and then moves continuously through rubber pinch rollersto a retort under flowing hydrogen. The retort is housed in a clam-shellfurnace that comprises a four-foot heated zone held at 730° Celsius. Thesteel fiber mat emerges into air and is cooled before traveling to anoxidative furnace operating under flowing air. Alternatively, thefurnaced steel fiber mat can be fabricated into a soot trap form factorat this stage followed by oxidative furnacing of the fabricated part.The oxidative furnace is heated to about 800° Celsius to about 950°Celsius for continuous processing of a fiber mat. The steel fiber mat isexposed to the oxidative furnace for about 5 to about 15 minutes. Astatic furnace is used to oxidize pre-formed parts.

The product (i.e., mat of fibers) that emerges from the oxidativefurnace can be coiled and cut to a desired length for fabrication into asoot trap. Once oxidized and fabricated, the formed soot trap componentcan be impregnated with catalytic agents and calcined using methodsknown in the art. For example, a simple catalytic soot trap (suitable asa test prototype) can be produced by taking the fibrous material afterit emerges from the oxidative furnace, coiling it, cutting the coiledmaterial, folding the coiled and cut material upon itself, rolling thefolded material into a cylindrical shape and then sliding thatcylindrical body into a 0.86 inch ID stainless steel tube cut to a3-inch length. An aqueous solution of a platinum group metal(s) may thenbe impregnated into the formed soot trap and the form calcined.

In one aspect of the invention, the mat of thin steel fibers thatemerges from the oxidative furnace is impregnated with a solution orsuspension comprising a platinum group metal catalyst and then driedand, optionally, further heat treated to form the final catalyticfibrous material.

As used herein, the term “platinum group metal catalyst” means anyplatinum group metal compound or complex, which, upon calcination or useof the catalyst decomposes or otherwise converts to a catalyticallyactive form. Water soluble compounds or water dispersible complexes aswell as organic soluble or dispersible compounds or complexes of one ormore platinum group metals may be utilized as long as the liquid used toimpregnate or deposit the catalytic metal compounds onto the pluralityof thin coated steel fibers does not adversely react with the catalyticmetal or its compound or complex or the other components of thecatalytic material, and is capable of being removed from the catalyst byvolatilization or decomposition upon heating. In some instances, thecompletion of the removal of the liquid may not take place until thecatalyst is placed into use and subjected to the high temperaturesencountered during operation.

Typically, aqueous solutions of soluble compounds or complexes of theplatinum group metals are preferred. For example, some of the compoundsthat may be used in the fibrous material of the instant disclosureinclude: gold (III) acetate, hydrogen tetrachloroaurate (III), ammoniumhexachloroiridate (IV), iridium (III) chloride hydrate, ammoniumtetrachloropalladate (II), palladium (II) nitrate, ammoniumtetrachloroplatinate (II), dihydrogen hexachloroplatinate (IV)(chloroplatinic acid), tetraamineplatinum (II) nitrate, rhodium (III)chloride hydrate, potassium pentachlororhodate (III), rhodium nitrate,ruthenium (III) chloride, and pentaaminepyridineruthenium (II)tetrafluoborate. Additional compounds may be added as cocatalyticagents, promoters, or as modifiers along with the platinum group metalcompounds. Separate impregnation steps may be necessary for certainadded compounds that might react to form precipitates with the platinumgroup metal compounds. Examples of non-platinum group metalpromoter/cocatalytic agent compounds include ammonium hexanitrocerrate(IV) and manganese (II) nitrate.

Once impregnation of the product (i.e., mat of steel fibers) thatemerges from the oxidative furnace with the solution or suspensioncomprising a platinum group metal catalyst is complete, the product iscalcined to convert the platinum group metal of the platinum group metalcatalyst to a well dispersed form, which is either catalytically activeor transforms to an active form in use.

After impregnation is complete, a filter may be formed from the fibrousmaterial that is suitable for partial capture of soot particles fromdiesel engine exhaust. Some filter designs are known in the art for thispurpose. These typically feature aspects such as partial bypass channelsthat enable approximately even distribution of captured particlesthroughout the depth of the filter element and serve to reduce pressuredrop across the filter. After final treatment, fiber mats typically arecompressed into relatively dense beds (of predefined densities) aroundthe bypass structures, which can be corrugated metal spacers, tubularpipes, grooves in the housing, or similar features. The assembly then isfitted into a shroud of appropriate dimensions for attachment to engineexhaust manifolds. When in use, periodic or continuous catalyticallyinitiated oxidation of embedded soot particles, under appropriate leanburn conditions, allows the filter element to destroy embedded soot andto reduce pressure drop caused by particulate buildup within the filterover time.

In another aspect of the disclosure, a filter is formed from the productthat emerges from the oxidative furnace in a similar fashion to thatdescribed above. The filter (as an individual part) is then impregnatedwith a solution or suspension comprising a platinum group metal catalystand, optionally, a promoter or co-catalytic agent, then heat treated asis appropriate to convert the catalyst precursors to active forms.

EXAMPLE 10 Laboratory Preparation and Characterization of CatalystSupport Fibers

Stainless steel grade 430 fibers of elliptical shape in the diameterrange of approximately 125 to 220 micron were supplied by RibbonTechnology Corporation in the form of a loosely pressed mat. The mat wascut into eight inch strips approximately 3 inches wide, washed withacetone, and air dried. Each of several mat strips then were mountedinto a laboratory-scale falling film dip coating machine and coated witha slurry composition consisting of 57.8% by weight aluminum metal powder(about 3 micron average particle size), 4.2% by weight methylacrylate-based binder, and the balance acetone solvent. The coatingslurry, prepared in a 900 mL batch, had been ball-milled overnight priorto coating using approximately 150 mL of ¼ inch diameter stainless steelballs as grinding/mixing media. After coating, each mat was hungvertically in a hot air stream to dry, then stapled or spot welded to a2 mil thick low carbon steel foil leader. The composite assembly ofleader and fiber mat was passed through a 4 foot furnace, which had beenfitted with a steel retort, held at 710° C. under flowing hydrogen at afeed rate of 6 feet per minute. The furnaced mat was cooled in air andcut into various aliquots used for further work. In some cases, thefurnaced, aluminized fibers first were formed into compressed cylinderssuitable for filtration prior to further processing, and in other cases,the loosely pressed mats were oxidized directly, as described below,then formed.

In a first oxidation stage, the furnaced material was heated in air in astatic oven ramped at 10° C. per minute between room temperature and620° C., then held at 620° C. for 1.5 hours. In a second stage, the oventemperature was increased from 620° C. to 850° C. at a ramp rate of 8°C./minute then soaked at 850° C. for 20 minutes. The sample was thencooled to room temperature in air slowly, and held within the oven as itcooled. Oxidized fibers increased in weight by about 1% during thistreatment. Oxidized fibers were characterized by SEM, EDS, and TGAtechniques and some aliquots tested as catalyst supports in the form offiber mats. FIG. 4 shows Scanning Electron Microscope images of thealuminum coated and oxidized fibers of this example compared tountreated fibers, as received. FIG. 4A shows the untreated controlfibers as received. FIG. 4B shows a low magnification image of treatedfibers after the oxidation step. FIG. 4C shows a high magnificationimage of treated fibers after the oxidation step. Surface analysis bythe EDS technique corresponding to one of the images of the treated andoxidized fibers shows an oxygen to aluminum atomic ratio of 1.5,consistent with a surface composition rich in aluminum oxide.

The fiber mat catalyst supports produced above can be infiltrated withsolutions or suspensions of catalyst materials (e.g., platinum groupmetal or “PGM” catalyst materials) and then dried to form catalyticallyactive bodies. Alternatively, we contemplate that other methods known inthe art may be suitable to load active catalytic species onto thesupport surfaces (e.g., chemical vapor deposition methods orsupercritical precipitation methods).

Still other objects and advantages of the present disclosure will becomereadily apparent to those skilled in the art from the preceding detaileddescription, wherein it is shown and described in preferred embodiments,simply by way of illustration of the best mode contemplated. As will berealized the disclosure is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the spirit or scope of the invention asset forth in the claims. Accordingly, the description is to be regardedas illustrative in nature and not as restrictive.

The term “comprising” (and its grammatical variations) as used herein isused in the inclusive sense of “having” or “including” and not in theexclusive sense of “consisting only of” The term “consisting essentiallyof” as used herein is intended to refer to that which is explicitlyrecited along with what does not materially affect the basic and novelcharacteristics of that recited or specified. The terms “a” and “the” asused herein are understood to encompass the plural as well as thesingular.

We claim:
 1. A method of producing a structured catalyst comprising: (a)preparing a slurry comprising one or more metal powders, includingaluminum; (b) coating a metal substrate, or a mat of metal fiber or awoven metal fiber assembly, with said slurry; (c) subjecting the coatedmetal substrate, coated metal fiber mat or coated woven metal fiberassembly to heat under an inert or reducing atmosphere wherein at leastone of the one or more metal powders melts and interdiffuses into thesurface of the metal substrate, or metal fiber mat or woven metal fiberassembly; (d) leaching the coated metal substrate or coated metal fibermat or coated woven metal fiber assembly obtained in step (c) in acaustic solution; (e) bathing the coated metal substrate, coated metalfiber mat or coated woven metal fiber assembly obtained in step (d) in achelating acid solution; (f) passivating the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly obtained instep (e); and (g) optionally abrading the surface of the coated metalsubstrate obtained in step (f).
 2. The method of claim 1, furthercomprising the following additional step to form a catalyst support: (h)subjecting the coated metal substrate, coated metal fiber mat or coatedwoven metal fiber assembly obtained after step (f) or step (g) to heatin an oxygen containing atmosphere for an additional period of time. 3.The method of claim 2, wherein the coated metal substrate, coated metalfiber mat or coated woven metal fiber assembly, after step (h),comprises an aluminum oxide coating as an outer layer and said methodcomprises the following additional step: impregnating at least a portionof the aluminum oxide outer layer with one or more catalytic substances.4. The method of claim 1, further comprising forming the coated metalsubstrate, coated metal fiber mat or coated woven metal fiber assemblyinto a catalyst structure before leaching.
 5. The method of claim 1,further comprising forming the coated metal substrate, coated metalfiber mat or coated woven metal fiber assembly into a catalyst structureafter passivating or after abrading, if abrasion is performed.
 6. Themethod of claim 1, wherein in step (c), the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly is subjectedto heat in a reducing atmosphere.
 7. The method of claim 1, wherein themetal of the metal substrate, mat of metal fiber or woven metal fiberassembly is either: (a) primarily nickel; or (b) a grade 430 stainlesssteel.
 8. The method of claim 1, wherein, in addition to said aluminum,the one or more metal powders are selected from the group consisting ofZr, V, Cr, Co, Ti, W, Nb, Mo, and Ta.
 9. The method of claim 1, wherein,in step (c), the coated metal substrate, coated metal fiber mat orcoated woven metal fiber assembly is subjected to a temperature of about650° C. to 950° C.
 10. The method of claim 1, wherein the acid ismineral acid or a carboxylic acid.
 11. The method of claim 1, whereinthe coated metal substrate is passivated using a solution comprisingH202.
 12. The method of claim 1, wherein the one or more metal powdersinclude powders made from alloys of two or more metals.
 13. The methodof claim 12, wherein said alloys include alloys made from Ni and Zr andalloys made from Ni and Cr.
 14. A method of producing a structuredcatalyst support comprising: (a) preparing a slurry comprising one ormore metal powders, including aluminum; (b) coating a metal substrate,or a metal fiber mat or a woven metal fiber assembly, with said slurry;(c) subjecting the coated metal substrate, coated metal fiber mat orcoated woven metal fiber assembly to heat under an inert or reducingatmosphere wherein at least one of the one or more metal powders meltsand interdiffuses into the surface of the metal substrate, or metalfiber mat or woven metal fiber assembly; and (d) subjecting the coatedand heat treated metal substrate, metal fiber mat or woven metal fiberassembly to heat in an oxygen containing atmosphere for an additionalperiod of time.
 15. The method of claim 14, further comprising formingthe coated metal substrate, coated metal fiber mat or coated woven metalfiber assembly into a catalyst structure after step (c).
 16. The methodof claim 14, further comprising forming the coated metal substrate,coated metal fiber mat or coated woven metal fiber assembly into acatalyst structure after step (d).
 17. The method of claim 14, whereinthe coated and heat treated metal substrate, metal fiber mat or wovenmetal fiber assembly obtained from step (d), comprises an aluminum oxidecoating as an outer layer and said method comprises the followingadditional step: impregnating at least a portion of the aluminum oxideouter layer with one or more catalytic substances.